ABSTRACT Chloride channels play crucial roles in many aspects of cell physiology. The genes encoding these channels are the loci for several human diseases relevant to the NIDDK. Understanding the structure and function of these proteins, and development of pharmaceutical agents targeting them, relies upon the availability of specific, high-affinity probes. The goal of this proposal is to characterize a novel peptide inhibitor which interacts with high affinity with the CFTR chloride channel. CFTR is defective in the lethal genetic disease, Cystic Fibrosis (CF), and also plays an important role in polycystic kidney disease (PKD) and secretory diarrhea. Peptide toxins from animal venom are among the most selective and useful tools for the study of ion channels; however, until now, no peptide toxins have been found that interact with anion channels of known molecular identity. This laboratory recently isolated a peptide toxin that inhibits CFTR. The novel toxin, GaTx1, inhibits CFTR in a state-dependent manner by locking channels into a long closed state. Hence, GaTx1 represents a quantum advance in how we can approach structure/function studies in CFTR, compared to the structural probes currently available. The present application proposes a series of objectives to characterize GaTx1, in three aims as follows. Aim 1 is to characterize the wildtype toxin by: determining kinetics of inhibition using single-channel patch clamp and macropatch recording, determining effects on ATP binding and hydrolysis by purified CFTR cytosolic domain polypeptides, and asking whether GaTx1 inhibits the conformational change underlying gating of the channel pore itself. Aim 2 is to localize the toxin's binding site by a series of independent studies using electrophysiological and biochemical approaches followed by site-directed mutagenesis. Aim 3 is to identify determinants of activity by mutating the toxin itself, leading to identification of the interacting surfaces. The approach takes advantage of a list of highly qualified collaborators with expertise complementary to that of the PI's lab. This work will provide the unique opportunity to use the GaTx1 toxin as a research tool, and also will likely aid in the design of novel therapeutics for CF, PKD, secretory diarrhea, and other pathologies that involve CFTR.